Polymers containing in the molecule a group which has high hydrophilicity and readily forms a hydrogen bond, such as poly(meth)acrylic acid and polyvinyl alcohol, are known as gas-barrier polymers. Although a film formed solely from such a polymer exhibits excellent gas (e.g., oxygen)-barrier properties under dry conditions, the film exhibits very poor gas (e.g., oxygen)-barrier properties under high humidity, because of the hydrophilic property of the polymer. In addition, such a polymer has poor resistance to moisture or hot water (e.g., the polymer dissolves in hot water), and such poor moisture or hot water resistance imposes a limitation on the industrial use of the polymer as a gas-barrier resin. The moisture or hot water resistance of poly(meth)acrylic acid or polyvinyl alcohol is described in Comparative Examples of Japanese Patent Application Laid-Open (kokai) No. 06-220221 (U.S. Pat. No. 5,552,479) filed by the present inventors.
In an attempt to solve the aforementioned problems, the present inventors have proposed a film product exhibiting excellent gas-barrier properties even at high humidity, the film product being produced by subjecting, to thermal treatment under specific conditions, a film formed from the following mixture: a mixture of poly(meth)acrylic acid and polyvinyl alcohol (Japanese Patent Application Laid-Open (kokai) No. 06-220221), a mixture of partially neutralized poly(meth)acrylic acid and polyvinyl alcohol (Japanese Patent Application Laid-Open (kokai) No. 07-102083 (U.S. Pat. No. 5,574,096)), or a mixture of a sugar, and poly(meth)acrylic acid or partially neutralized poly(meth)acrylic acid (Japanese Patent Application Laid-Open (kokai) No. 07-165942 (U.S. Pat. No. 5,498,662)).
Meanwhile, researchers other than the present inventors have proposed, for example, a gas-barrier resin composition characterized by prepared through chemical treatment, by means of heat and active energy beams, of a composition containing a carboxyl-group-containing resin which readily forms a hydrogen bond (specifically, a poly(meth)acrylic acid polymer), a hydroxyl-group-containing resin which readily forms a hydrogen bond (specifically, a sugar), and an inorganic layered compound (Japanese Patent Application Laid-Open (kokai) No. 10-231434); and a resin composition characterized by containing a water-soluble polyacrylic acid compound, polyvinyl alcohol, and an inorganic layered compound, which composition exhibits excellent gas barrier properties at a relative humidity (Japanese Patent Application Laid-Open (kokai) No. 11-246729).
Regarding films formed solely from a polyacrylic acid polymer, for example, Japanese Patent Application Laid-Open (kokai) No. 2001-19782 discloses a film which is formed from partially neutralized polyacrylic acid, which contains no polyvinyl alcohol, and which exhibits improved oxygen gas-barrier properties through electron beam irradiation.
Although a film produced through any of the aforementioned known techniques exhibits excellent gas-barrier properties at high humidity, the film has unsatisfactory resistance to high-temperature water vapor or hot water. These known techniques are focused on chemical treatment of a mixture of a poly(meth)acrylic acid polymer and a polyalcohol polymer, or chemical treatment of a polyacrylic acid polymer, by means of heat, active energy beams, electron beams, or the like. However, when such treatment is carried out, the resultant film is not easily dissolved in an acid or an alkali, and therefore, separation/recovery or recycling of the film becomes difficult upon disposal thereof.
In an attempt to enhance the high-temperature water vapor resistance or hot water resistance of a film formed from a poly(carboxylic acid) polymer and a polyalcohol polymer, the present inventors have further proposed a resin composition containing a reaction product formed from a poly(meth)acrylic acid polymer, a polyalcohol polymer, and a polyvalent metal (Japanese Patent Application Laid-Open (kokai) No. 10-237180 (U.S. Pat. No. 6,143,384)). Specifically, Japanese Patent Application Laid-Open (kokai) No. 10-237180 discloses a gas-barrier resin composition characterized by having a chemical structure including an ester bond formed through reaction between a poly(meth)acrylic acid and a polyalcohol, and an ionic bond formed between the poly(meth)acrylic acid and polyvalent metal ions. This publication discloses that when the amounts of ester bonds and ionic bonds are limited to fall within specific ranges, the resultant film exhibits resistance to high-temperature water vapor or hot water. Japanese Patent Application Laid-Open (kokai) No. 10-237180 also discloses a method for forming an ester bond between a poly(meth)acrylic acid and a polyalcohol by means of thermal treatment; and a method for forming an ionic bond between the poly(meth)acrylic acid and polyvalent metal ions by immersing a mixture of the thermally treated poly(meth)acrylic acid and polyalcohol in water containing a polyvalent metal compound.
The present inventors have also proposed a film containing a poly(carboxylic acid) polymer and a polyalcohol polymer, which film is produced by means of a simpler method and exhibits enhanced resistance to high-temperature water vapor or hot water (Japanese Patent Application Laid-Open (kokai) No. 2000-931 (European Patent No. 1086981A1)). Specifically, Japanese Patent Application Laid-Open (kokai) No. 2000-931 discloses a film produced by applying a layer containing a metallic compound onto the surface of a molded product layer formed of a mixture of a poly(meth)acrylic acid polymer and a polyalcohol polymer. This publication discloses that the aforementioned method readily produces a film exhibiting enhanced gas (e.g., oxygen)-barrier properties and enhanced resistance to high-temperature water vapor or hot water.
Japanese Patent Application Laid-Open (kokai) Nos. 10-237180 and 2000-931 disclose that a polyvalent metal compound is caused to act on a mixture of a poly(meth)acrylic acid polymer and a polyalcohol polymer, and that the resultant gas-barrier resin composition or the resultant film exhibits resistance to high-temperature water vapor or hot water. However, in order to impart sufficient oxygen gas-barrier properties, high-temperature water vapor resistance, and hot water resistance to the gas-barrier resin composition or film, the poly(meth)acrylic acid polymer-polyalcohol polymer mixture must be chemically treated by means of a predetermined treatment such as thermal treatment. In the case where the mixture is incompletely treated; i.e., the thus-treated mixture contains polyalcohol, even when a polyvalent metal compound is caused to act on the mixture, the resultant film may fail to exhibit sufficient oxygen gas-barrier properties and resistance to high-temperature water vapor or hot water. Thus, the poly(meth)acrylic acid polymer-polyalcohol polymer mixture must be chemically treated by means of, for example, heat. However, when such treatment is carried out, the resultant film is not easily dissolved in an acid or an alkali, and therefore, separation/recovery of the film becomes difficult upon disposal thereof.
Next will be described techniques for causing a polyvalent metal compound to act solely on a poly(carboxylic acid) polymer, which techniques do not necessarily relate directly to the aforementioned film-related technical field. A mixture of polyacrylic acid and a polyvalent metal compound is publicly disclosed by, for example, the following academic publications: A. Ikegami, Journal of Polymer Science, Vol. 56, p. 133 (1962), Yasuo Goto, SEN'I GAKKAISI, Vol. 55, No. 11, pp. 522-527 (1999), and L. E. Nielsen, Polymer Engineering and Science, Vol. 9, No. 5 (1969). A. Ikegami, et al. have reported formation of precipitates by causing a divalent metal compound to act on an aqueous sodium polyacrylate solution. As has been reported, polyacrylic acid readily reacts with a polyvalent metal compound in an aqueous solution. Yasuo Goto, et al. and L. E. Nielsen, et al. have reported a method in which a polyvalent metal compound is reacted with an aqueous polyacrylic acid solution, and the thus-formed polyacrylic acid polyvalent metal salt is subjected to compression molding at very high pressure. As has been reported, a polyacrylic acid polyvalent metal salt can be readily formed by the mediation of an aqueous polyacrylic acid solution. However, difficulty is encountered in forming a uniform film-like molded product by means of this method. In addition, molding of the thus-formed polyacrylic acid polyvalent metal salt requires application of high pressure.
Attempts have been made to form a film from a polyacrylic acid polyvalent metal salt, as described in, for example, the following academic publication: A. Claudio Habert, Journal of Applied Polymer Science, Vol. 24, pp. 489-501 (1979). A. Claudio Habert, et al. have reported a method in which an aqueous polyacrylic acid solution is spread on a glass plate, and the aqueous-solution-spread plate is immersed in a polyvalent metal compound solution, to thereby react polyacrylic acid with the polyvalent metal. This method successfully produces a uniform film under specific conditions when a compound of a specific metal (e.g., Al) is employed. However, as has been reported, difficulty may be encountered in forming a uniform film, depending on the type of a polyvalent metal compound to be employed or reaction conditions, since this method employs reaction between an aqueous polyacrylic acid solution and a polyvalent metal compound solution (i.e., solution-solution reaction).
Next will be described techniques relating to a coating liquid prepared by causing a polyvalent metal compound to act on a poly(carboxylic acid) polymer, which techniques also do not necessarily relate directly to the aforementioned film-related technical field. For example, Japanese Patent Application Laid-Open (kokai) No. 54-82416 discloses a pigment dispersant for forming coated paper, the dispersant containing an aqueous solution prepared by reacting a compound of a polyvalent metal (e.g., calcium, aluminum, or zinc) with a polymer or copolymer formed from an α,β-monoethylenic unsaturated carboxylic acid, and/or an alkali metal salt or ammonium salt of the polymer or copolymer. Japanese Patent Application Laid-Open (kokai) No. 5-263046 discloses a method for producing a coating agent composition, which method is characterized in that a film-forming agent and/or a plasticizer is added to an emulsion containing a polymer having an acid value of 20 to 200, the polymer being produced through polymerization of an ethylenic unsaturated monomer; and subsequently a polyvalent metal compound is added in an amount of 0.05 to 0.9 chemical equivalents (hereinafter may be referred to simply as “eq”) on the basis of the acid value of the polymer, thereby reacting the compound with the polymer. Japanese Patent Application Laid-Open (kokai) No. 8-176316 discloses an alkali-soluble film and an alkali-soluble coating agent, each of which contains a polyvalent metal and a polymer produced through non-aqueous polymerization of an α,β-unsaturated carboxylic acid monomer and at least one vinyl-group-containing monomer. In these known techniques, a coating liquid is prepared through, for example, addition of an alkali metal or ammonia to an aqueous poly(carboxylic acid) polymer solution, employment of a poly(carboxylic acid) polymer in the form of emulsion, or employment of a copolymer formed from an α,β-unsaturated carboxylic acid monomer and a vinyl-group-containing monomer. However, when a film is formed only through application of such a coating liquid onto a support, the resultant film fails to exhibit satisfactory gas (e.g., oxygen)-barrier properties and resistance to high-temperature water vapor or hot water.
Conventional techniques relating to, for example, a gas-barrier film formed from a poly(carboxylic acid) polymer have been described so far with reference to the laid-open patent publications and academic publications. In summary, in the case where a film exhibiting excellent gas (e.g., oxygen)-barrier properties even at high humidity and having resistance to high-temperature water vapor or hot water is to be formed from a poly(carboxylic acid) polymer, there may be employed a method in which a mixture of a poly(carboxylic acid) polymer and a polyalcohol polymer is chemically modified through, for example, thermal treatment, and a polyvalent metal is caused to act on the resultant mixture. However, in a film formed through this method, an ester bond is formed between the poly(carboxylic acid) polymer and the polyalcohol polymer through such a chemical treatment. Therefore, the film is not easily dissolved in an acid or an alkali, and separation/recovery of the film becomes difficult. Meanwhile, there has been described a technique in which a polyvalent metal compound is caused to act solely on a poly(carboxylic acid) polymer, without mixing of the polymer with a polyalcohol polymer and without chemical treatment of the polymer through, for example, thermal treatment. However, even when a film is formed from a poly(carboxylic acid) polymer through such a technique, a limitation is imposed on the industrial use of the film.
Objects of the present invention are to provide a film which can be readily produced on an industrial scale at low cost, which exhibits excellent gas (e.g., oxygen)-barrier properties even in a high-humidity atmosphere, which has water resistance such that the appearance, shape, and gas-barrier properties are not impaired by neutral water, high-temperature water vapor, or hot water, which is easily dissolved in an acid and/or an alkali, and which exhibits good disposability; i.e., separation/recovery of the film can be carried out upon disposal thereof; to provide a laminate comprising the film; to provide a method for producing the film or the laminate; and to provide use of the film or the laminate.